1. Field of the Invention
The present invention relates to a semiconductor device, particularly to a sense amplifier used in a semiconductor device especially including a thin film transistor.
2. Description of the Related Art
In a semiconductor memory apparatus, a sense amplifier is used for reading stored data. The construction and the operation of an example of a conventional sense amplifier will be described with reference to FIG. 26. In FIG. 26, gates of NMOS transistors MN1 and MN2 are connected to a first input terminal IN1 and a second input terminal IN2, respectively, corresponding to a pair of bit lines. Both sources of the PMOS load transistors MP1 and MP2 are connected to a power source VDD. The PMOS load transistors MP1 and MP2 have a common gate, which is further connected to a drain of the MP1. The drain of the MP1 is connected to the drain of the MN1. The drain of MP2 is connected to the drain of the MN2 and an output terminal OUT. The MN1 and the MN2 have a common source, which is connected to the drain of an NMOS transistor MN3 functioning as a direct current source. The gate and the source of the MN3 are connected to a bias power source V_BIAS and a power source GND, respectively. The sense amplifier shown in FIG. 26 is included in a current mirror type differential amplifier.
In a semiconductor memory apparatus, when stored data is read out, differential potentials, which are opposite in height, occur in a pair of bit lines depending on whether the data is xe2x80x9cHixe2x80x9d or xe2x80x9cLoxe2x80x9d. The sense amplifier shown in FIG. 26 detects a small potential difference of signals in bit line sent to the first input terminal IN1 and the second input terminal IN2. Then the sense amplifier amplifies and output the result. In other words, when the potential of IN2 is larger than the potential of the IN1, the output terminal OUT outputs xe2x80x9cLoxe2x80x9d. On the other hand, when the potential of the IN2 is smaller than the potential of the IN1, the output terminal OUT outputs xe2x80x9cHixe2x80x9d. In this way, the sense amplifier is used for reading data stored in the semiconductor memory apparatus.
Recently, an active matrix type image display apparatus using an image display device, especially, a thin film transistor (called TFT hereinafter) having a semiconductor thin film on a glass substrate, has been widely spread. The active matrix type image display apparatus (called image display apparatus, hereinafter) using a TFT has hundreds and thousands to several millions TFT""s arranged in matrix and controls charges of pixels. Furthermore, a TFT technology (such as polysilicon TFT technology) is evolving recently in which not only pixel TFT is used to construct a pixel but also TFT""s are used for forming a drive circuit, a memory circuit, a control circuit and even CPU at the same time.
However, according to the current TFT technology, the variation in characteristic of transistors is larger than that of the technology in which at least one transistor is formed on a single crystal Si substrate. This means that the circuit in the conventional example shown in FIG. 26 is difficult to use in the present TFT technology. For example, it is assumed that threshold values of the NMOS transistors MN1 and MN2 are 1.0 V and 1.5 V, resulting in a difference of 0.5 V. When the potential of the second input terminal IN2 is 0.2 V larger than the potential of the first input terminal IN1, the output OUT should be xe2x80x9cLoxe2x80x9d. However, in reality, the output OUT is xe2x80x9cHixe2x80x9d, resulting in a wrong operation. This is critical when the conventional circuit is used as a read circuit for a dynamic random access memory (DRAM).
When the conventional circuit is used as a read circuit for a static random access memory (SRAM), the potential difference between the input terminals is increased over time. At last, the potential difference absorbs the variation in threshold values of the NMOS transistors MN1 and MN2. Thus, the possibility of causing the wrong operation is decreased. However, the large input potential difference absorbing the threshold variation takes time to obtain. As a result, the reading time becomes longer.
Accordingly, in view of these problems, it is an object of the present invention to provide a sense amplifier suppressing an effect of the threshold variation. It is another object of the present invention to provide a sense amplifier including TFT""s having good characteristics.
In order to achieve these objects, a sense amplifier according to the present invention has a construction as follows:
According to one aspect of the present invention, there is provided a sense amplifier for detecting a potential difference of signals input to a first input terminal and a second input terminal, including a first means for applying voltages corresponding to threshold voltages of first and second transistors to gate-source voltages of the first and second transistors, and a second means for transferring signals input to the first and second input terminals to gates of the first and second transistors, wherein a threshold variation of the first and second transistors are corrected.
The first and second transistors may have a common source, which is connected to a first power source through a first switch.
The first and second transistor may have drains connected to a second power source through second and third switches, respectively. The drain of the first transistor may be connected to a second power source through a second switch and a first resistor, and the drain of the second transistor may be connected to the second power source through a third switch and a second resistor.
Preferably, the drain of the first transistor is connected to a second power source through a second switch and a third transistor. The drain of the second transistor may be connected to the second power source through a third switch and a fourth transistor. Both gates of the third and fourth transistors may be connected to the drain of the third transistor.
In this case, the first means may include a first switching means for controlling conduction/nonconduction between gates and drains of the first and second transistors, respectively, a second switching means for controlling charging or discharging charges to the drains of the first and second transistors, respectively, and a third switching means for controlling charging or discharging charges to the sources of the first and second transistors.
The second means may be implemented through capacitors between the first and second input terminals and gates of the first and second transistors, respectively. Preferably, the second means may be implemented by having capacitors and switches connected in series between the first and second input terminals and gates of the first and second transistors, respectively, and by having fourth and fifth switches for controlling connection with a third power source at connection nodes of the two pairs of the capacitors and switches, respectively.
According to another aspect of the present invention, there is provided a sense amplifier including a first transistor having a gate connected to a first input terminal, a second transistor having a gate connected to a second input terminal, a third transistor having a source connected to a first power source, and a fourth transistor having a source connected to the first power source. In this case, sources of the first and second transistors are connected to each other. The sense amplifier detects a potential difference of two signals input to the first and second input terminals. The sense amplifier further includes a first means for causing gate-source voltages of the third and fourth transistors to store voltage corresponding to threshold values of the third and fourth transistors. In this case, a threshold variation of the third and fourth transistors is corrected.
The first means may have a first switch between the common source of the first and second transistors and a second power source, a second switch between the gate and the drain of the third transistor, a third switch between the gate and the drain of the fourth transistor, a first capacitor between the gate and the drain of the third transistor, a second capacitor between the gate of the fourth transistor and the drain of the third transistor, a first node where the drains of the first and third transistors are connected to each other, and a second node where the drains of the second and fourth transistors are connected to each other.
Preferably, the first means has a transistor between the source of the first transistor and a second power source, the transistor having a gate connected to a bias power source, a second switch between the gate and the drain of the third transistor, a third switch between the gate and the drain of the fourth transistor, a first capacitor between the gate and the drain of the third transistor, a second capacitor between the gate of the fourth transistor and the drain of the third transistor, a fourth switch between the drain of the first transistor and the drain of the third transistor, and a fifth switch between the drain of the second transistor and the drain of the fourth transistor.
Alternatively, the first means may have a transistor between the source of the first transistor and a second power source, the transistor having a gate connected to a bias power source, a second switch between the gate and the drain of the third transistor, a third switch between the gate and the drain of the fourth transistor, a first capacitor between the gate and the drain of the third transistor, a second capacitor between the gate of the fourth transistor and the drain of the third transistor, a fourth switch between the drain of the first transistor and the drain of the third transistor, a fifth switch between the drain of the second transistor and the drain of the fourth transistor, a sixth switch between the drain of the third transistor and a third power source, and a seventh switch between the drain of the fourth transistor and the third power source.
The sense amplifier is constructed by using a thin film transistor. A semiconductor film functioning as a semiconductor active layer of the thin film transistor is crystallized by laser anneal method using serial oscillating laser light.
Accordingly, the present invention can be applied widely to electronic apparatus in all fields.